The invention relates to a colour cathode ray tube and more particularly to the frame/colour selection mask system disposed inside the glass envelope of the tube. The invention finds its application in any type of tube comprising a colour selection mask and is more particularly adapted to tubes whose mask is held in tension by the frame to which it is secured.
Conventional cathode ray tubes comprise a colour selection mask situated a precise distance from the inside of the glass faceplate of the tube, on which faceplate are deposited arrays of red, green and blue luminophores. Under the influence of three electron beams each corresponding to a specified primary colour, the arrays of luminophores allow the reproduction of images on the screen, the mask allowing each specified beam to illuminate only the luminophore of the corresponding colour.
The colour selection mask must be disposed and held during the operation of the tube in a precise position inside the said tube. The holding functions are carried out by virtue of a generally very rigid rectangular metal frame to which the mask is conventionally welded. The frame/mask assembly is mounted in the faceplate of the tube by virtue of elastic suspension means comprising at least three metal parts welded to the frame, each of the said parts comprising a spring-forming and apertured end intended to be engaged in one of the metal studs integral with the glass faceplate. Two arrangements of the suspension means are conventionally used: either in the middles of the sides of the frame, or at its corners.
The current tendency is for tubes whose faceplate is ever more flat, moving towards totally flat faceplates. To make tubes comprising such a faceplate entails a technology consisting in using a flat mask, held in tension in at least one direction. Such structures are described for example in U.S. Pat. No. 4,827,179.
The colour selection mask consists of a metal foil of very small thickness, generally tensioned and secured to a frame of considerable mechanical strength in such a way as to maintain the tension of the mask during the operation of the tube.
Since the mask intercepts more than half the electrons emitted by the electron gun of the tube during the operation of the latter, its temperature rises rapidly, causing the expansion of at least certain zones of the said mask; this results in a phenomenon of reduction in its tension and in its flatness. The mask no longer being flat, it follows that the points of impingement of the three electron beams on the screen of the tube no longer correspond to the ad hoc lines of luminescent materials, thus causing a decoloration of the image reproduced on the said screen.
A first solution consists in using a mask made from a material of very low coefficient of thermal expansion such as, for example, Invar for which this coefficient is of the order of 1.25 10xe2x88x926/xc2x0 C. The mask will then expand very little during the operation of the tube, thus affording a guarantee of its flatness. Consequently, it will be possible to use a relatively low mask tension, for example of the order of 5 kg/mm2, to guarantee its flatness without fear of it becoming slack during the operation of the tube; the metal support frame will therefore have a mechanical strength in relation to the low tension applied.
A second solution consists in using a standard steel mask of larger coefficient of thermal expansion, of the order of 1.5 10xe2x88x925/xc2x0 C., which will have to be held flat with a much larger tension of possibly up to ten times the tension which has to be applied to an Invar mask. This necessitates the use of a holding frame whose mechanical strength is considerable, this mechanical strength of the frame being obtained through the considerable amount of material used. This solution then exhibits the drawbacks of high material costs and high weight.
The solution consisting in using a mask made of a material with a low coefficient of thermal expansion also exhibits a number of drawbacks, however:
The tension mask cannot be welded directly to the frame which holds it in tension unless the said frame is made from a material whose coefficient of thermal expansion is similar to that of the mask, and it cannot in particular be made from standard steel. This is because, during the phases of the tube manufacturing process, the frame/mask assembly is raised to temperatures above 400xc2x0 C.; at these temperatures the differential in the expansion of the materials where the mask is welded to the frame may cause local failures of the weld and hence local and random modifications to the tension of the mask, modifications giving rise to defects in the colours of the image reproduced on the screen of the tube.
This solution therefore necessitates the use of a frame and a mask whose coefficients of thermal expansion are similar and small, for example made of Invar; however, this type of material is much more expensive than standard steel, around three times more expensive, thereby very considerably increasing the cost of the frame and consequently of the manufactured tube.
There is a need to allow the use of a mask made of a material of low coefficient of thermal expansion associated with a frame made of conventional steel in such a way as to obtain a frame/mask assembly of low cost and low weight.
To do this, the frame/mask assembly according to the present invention comprises a colour selection mask and a substantially rectangular metal frame for supporting the mask and holding it in tension in at least one direction, the mask being made from a material whose coefficient of thermal expansion is less than the coefficient of thermal expansion of the material constituting the majority of the frame. Along at least two opposed sides of the frame are disposed two metal inserts extending along and secured to said opposed sides, and said metal inserts being made from a material whose coefficient of thermal expansion is similar to that of the mask, the mask being secured to the frame, for example by welding to the two metal inserts.